Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1998-09-02
2001-11-06
Milano, Michael J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06312467
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method of producing restructured bone, and more particularly to a method causing bone to bond to an implant containing a calcium phosphate component, and a method to control the restructuring of bone through the use of an implant containing calcium phosphate.
BACKGROUND ART
The numbers in brackets below refer to publications listed in the Appendix, the teachings of which are incorporated herein by reference.
When viewed from an engineering perspective the human skeleton is a marvelous engineering structure. For the long bones, for example, the cartilage-covered epiphyseal surfaces of articulating joints cannot carry heavy stresses [1]. So the joints are enlarged to reduce that stress and provide smooth articulating surfaces with extremely low friction. These bearings are supported by a thin layer of dense bone supported by trabecular bone, with the trabecular bone aligned in accordance with Wolff's law to convey the low-stress bearing forces to the dense, high-stress cortical bone of the diaphysis [2]. This remarkable structure is even more remarkable because, during growth, the dense ring of diaphysis bone just below the growth plate is continuously elongated, with cartilage calcifying below the growth plate and chondrocytes building cartilage above, all without loss of strength as the process operates to elongate the bones [3]. In this process the cell differentiation, and the vascularization and ossification mechanisms, operate continuously to produce the dense bone of the diaphysis, and still maintain the large-area bearing surfaces. The stiffness of the diaphysis, with its dense cortical bone forming a tube, is much greater than that of a solid rod of the same mass. The exterior of the epiphysis is a structural membrane. From a mechanical point of view this is an example of a marvelous engineering structure.
The physiological process mentioned above can also be approached from an engineering point of view. It can be seen that, in each part of the structure there is a supply of cells, nutrients, enzymes, and chemicals to provide for growth, repair, and remodeling. Using the long bone joint as an example, provision for cellular activity is essential for growth, repair, and remodeling. For growth to take place there must be a steady supply of the necessary nutrients. Mechanisms for controlling that supply are extraordinarily complex [4]. For example, at the growth ring, formation of cartilage is inhibited by its own growth, causing differentiation of chondrocytes into osteoblasts. The perichondrium becomes a periosteum. The chondrocytes hypertrophy and die, producing a collagenous matrix containing cavities left by the empty chondrocyte lacunae. Osteoblasts line the cartilage and synthesize osteoid. The cartilage begins to calcify. This mineralization reduces diffusion of nutrients and the osseous tissue contracts, producing dense osseous tissue of smaller diameter, that of the diaphysis. The mechanisms are not understood. For example, the mechanism of precipitation of hydroxyapatite is unknown [5]. Transport of calcium and phosphate ions proceed separately. Nucleation may take place homogeneously in matrix vesicles, or heterogeneously at collagen fibril voids [6]. Regardless of the mechanisms and the complexities, however, this can be considered analogous to an engineering system, something like the logistics of battle support. The necessary nutrients, mechanisms, and other factors must be provided in a timely manner for the tissue to remain healthy and strong. This can include electrical transport, the effects of stress, and many other factors. In order to have a successful prosthesis, then, all aspects of skeletal engineering must be considered. Unfortunately this has not been done.
Orthopedic research can be divided into two areas, clinical and academic. Clinical research includes the improvements and modifications of existing prostheses and surgical procedures to improve patient care. Outstanding surgeons often work with prosthesis manufacturers to modify the existing practice in attempts to improve the performance [7]. Recently this has included the introduction of beads, mesh, and other structures on the surface of metallic implants to provide for tissue ingrowth and assist in stabilization [8]. Unfortunately, a roughened implant can also cause tissue irritation and lead to failure. New ideas, such as the substitution of pure titanium for the 6Al4V alloy, have been tested in the clinic. In the dental area, for example, two tooth-root prostheses made of carbon have been introduced into clinical practice and subsequently withdrawn because of failure [9, 10]. Thus the industry often is doing its research in the clinic.
Academic research is conducted through government laboratories, private laboratories, and academia. Here the research is highly specialized. Each individual specialty is dominated by the theory and practice of that specialty. For example, new materials are often evaluated on the basis of specific cellular responses to those materials regardless of whether the material is successful [11]. For example, carbon/Teflon mesh has been tested as an orthopedic material despite the fact that it is easily crushed in one's fingers [12]. Also, only after other countries adopted the idea of osseous integration was the Branemark titanium implant for tooth roots accepted in the United States. Before that time osseous integration was not considered grounds for acceptance.
The special nature of academic research has been allowed to dictate the research results. For example, bone and tissue interactions are often modeled by finite element methods to determine stress distributions in the prosthesis and in the bone [13]. But bone is complex. Almost all of these efforts have modeled bone as a continuum. In fact, bone is not isotropic and not homogeneous. Even today most finite element analyses are conducted assuming cortical bone has one set of homogeneous properties and cancellous bone has another set of homogeneous properties [14]. Actual bone varies tremendously, and the structure of the bone must be considered. When this is not done the results may be wrong and misleading. An even more serious criticism of this research is that the bone changes in response to the prosthesis. Where stresses are very high, bone is resorped, distributing the stress and changing the geometry. This is not modeled, so the model is inaccurate as soon as the implant is in place.
The political desires of research funding also strongly affect academic research. There is a large funded effort to study orthopedic materials in vitro, to avoid in vivo studies. Programs in cell attachment and other areas are funded despite the fact that the correlation of in vivo with in vitro has not been established [15].
The research method of academia is to divide and conquer; to study one variable while keeping all others constant. This advances fundamental knowledge but does not consider the interaction of various factors. Real orthopedic implants require the simultaneous application of chemistry, basic biological science, physiology, anatomy, materials science, stress analysis, and systems analysis. The bottom line is successful orthopedic performance. This requires synthesis from all pertinent areas, identification of the important and the trivial factors, experimentation, and decision making. This is the essence of engineering design.
Each year in the United States about 250,000 total hips replacement surgeries are performed. And about 25,000 hip prosthesis replacements are performed. The number of replacements is expected to climb because the life expectancy of a hip is 5 to 15 years. Patients are living longer. There is a need for improved hip prostheses.
Tissue response is critical to the success of an implant. It is the tissue response that determines the life of the implant because the hard tissue is continuously rem
Dickstein , Shapiro, Morin & Oshinsky, LLP
Iowa State University & Research Foundation, Inc.
Milano Michael J.
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